I honestly assume it is a game bug, unless you have mods that allow elder pets to have babies, I have one that allows Elder Sim females to get pregnant, if I wanted them to be able to. I'm not sure if that expends to pet elders, as I've never looked that far in Story Progression to look. But Story Progression doesn't do anything non-included in the game pregnancies unless you activate them first. Do you use mods?